According to American Podiatric Medical Association, the pressure that a person feet sustain is approximately equal to one's weight while walking and as well as 3 to 3 times of one's weight while running. Taking a person weighing 60 Kg as an example, the overall pressure that one's feet sustain in a one day is as high as 600-1200 tons. It is thus likely to cause weariness and pains to the feet if the reactive pressure applied by the ground is only counteracted by a thin layer of foaming material, thereby damaging one's knees, waist or even spine after an extensive period of time.
To effectively relieve the pressure applied to one's feet, various shoe soles claiming to be equipped with air cushioning effect have been developed. Where the so-called elastic and shock-absorbing effects are able to support foot arch by means an arch chamber to mitigate pressure sustained by the feet and to improve foot comfort. For example, U.S. Pat. No. 5,218,056 discloses an elastic air cushion, where indents are formed at locations corresponding to one's metatarsus and calcaneus, in which indents sheet-like elastic air cushions are received. U.S. Pat. No. 4,918,838 further discloses an elastic air cushion structure, where a cylindrical shock-absorbing component is placed in a circular bore formed at the heel.
Though the above air cushioned shoes are capable of reducing the reactive pressure resulted from one's weight, their structure tends to be relatively complicated and result in dislocation of the shock-absorbing component after a period of time, thereby failing to provide an optimum contact between the shock-absorbing component and soles at a proper orientation, and resulting in discomfort similar to prolonged toe-tipping.
Furthermore, the Taiwan Patent Application No. 90215337 entitled “Composite Inner Sole with Hard and Soft Features” discloses a composite inner sole integrally formed of a material with different hardness by injection forming. The sole top of the inner sole is provided with a hard end portion and at least one soft end portion. The hard end portion and the soft end portion is progressively joined to each other. The soft end portion is pliable, flexible and deformable while the hard end portion is rigid, less flexible and deformable. However, the soft end portion is partially provided to the sole top so as to require high precision in manufacturing. Furthermore, the degree of wearing for the soft end portion is not comparable to the hard sole top, thereby resulting in discomfort similar to prolonged toe-tipping as the soft end portion is not located at the same level of the hard sole top after a period of time.